Battery system containing phase change material-containing capsules in interior configuration thereof

ABSTRACT

Provided is a battery system in which an interior part of a battery structure includes phase-change particles including a capsule and phase-change materials. The phase-change materials have a high latent heat of phase change at a specific temperature, and are encapsulated in the capsule. The capsule is made of an inert material. The battery system in accordance with the present invention can prolong a service life of the battery by inhibiting temperature elevation inside the battery under normal operating conditions without substantial effects on size, shape and performance of the battery, and further, can inhibit the risk of explosion resulting from a sharp increase in temperature inside the battery under abnormal operating conditions, thereby contributing to battery safety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 12/967,332 filed Dec. 14, 2010, which is acontinuation application of U.S. patent application Ser. No. 11/197,025filed Aug. 4, 2005 and issued as U.S. Pat. No. 7,931,979 on Apr. 26,2011, which claims priority to and the benefit of Korean PatentApplication No. 10-2004-0062049 filed on Aug. 6, 2004, all of which areincorporated by reference herein in their entirety

FIELD OF THE INVENTION

The present invention relates to a battery system including phase changematerial-containing capsules, otherwise referred to as “phase-changeparticles,” which are coated with conductive material to form coatedparticles. More specifically, the present invention relates to a batterysystem having prolonged service life and improved safety by inhibitingtemperature elevation during normal operation of the battery as well assharp increase of battery temperature due to abnormal operation, viaincorporation of a material having high latent heat of phase change(phase change material: PCM) contained in a capsule made of an inertmaterial into internal parts of the battery structure, for exampleelectrode active materials, current collectors, separators,electrolytes, inner or outer surfaces of the battery cases, interior orexterior parts of pouch cases of polymer cells, and outer structures ofbattery packs.

BACKGROUND OF THE INVENTION

Rapid growth of the portable electronics industry has led to increaseddemand for batteries, while a rise of internal temperature of thebatteries raises a great deal of problems. Typical problems associatedwith elevation of temperature inside the battery will be reviewedhereinafter.

For example, generation of heat upon charge/discharge of the batteryunder normal operating conditions leads to operation of the battery at atemperature higher than the outside thereof. Consequently, occurrence ofsuch a high temperature during operation of the battery results in rapiddegradation of the battery. Further, rapid elevation in internaltemperature of the battery under abnormal operating conditions is aleading cause of battery explosion.

Even though heat generation within a certain limit may be of help tooperation of the battery, temperatures outside a specific range and arapid increase of battery temperature are undesirable in terms ofservice life and safety of the battery.

As attempts to solve such problems, various methods have been developedwhich involve incorporation of flame retardants into certain structuralelements of the battery, or induces hardening of electrolytes when thebattery temperature is higher than a certain temperature, in order toprevent the risk of battery explosion due to sudden increases intemperature of the battery. However, these methods may be employed asmeasures capable of preventing battery explosion under abnormaloperation states, but are not designed to inhibit temperature elevationduring normal battery operation. Further, these methods aredisadvantageous in that the state of the battery is changed intoirreversible state and therefore the battery cannot be used any longer.

As such, there is an urgent need for development of techniques capableof prolonging the service life of the battery by inhibiting temperatureelevation inside the battery under normal operating conditions or atleast lowering an elevation rate of temperature, and capable of furtherimproving safety of the battery by inhibiting rapid increase of thebattery temperature.

Meanwhile, there are known techniques utilizing materials having highlatent heat of phase change or phase transfer which are designed forcertain applications. For instance, a technique is known which applieshigh-latent heat materials to garments, furnishings or the like so as toinduce gentle temperature changes therein, in spite of rapid temperaturechanges in the outside, thereby providing more comfortable environment.

In addition, some techniques, in which such high latent heatcharacteristics are applied to batteries, are also known in the relatedart. For instance, in order to prevent adverse effects on humans byinhibiting rapid temperature elevation of a battery as a power source inimplantable medical devices, International Publication No. WO 03/061032has proposed a method of installing a battery in a housing including ahigh-latent heat material, a method in which the high-latent heatmaterial is inserted in the form of heat absorbing mass inside a batterycase, and a method in which the battery is assembled by inserting thehigh-latent heat material between a cathode sheet, an anode sheet and aseparator sheet in the form of network-like endothermic mass. However,methods involving inserting the latent heat material, in the form ofseparate heat absorbing mass or network-like mass, into the interior ofthe battery case results in disadvantages such as increased size anddeteriorated performance of the battery. As a result, there remains aneed in the art for development of technology capable of solving suchproblems associated with battery size and performance together withtemperature elevation inside the battery as mentioned above.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide atechnique for simultaneously improving service life and safety of thebattery by inhibiting temperature elevation therein under normaloperation of the battery as well as a sharp increase of temperatureunder abnormal operation while minimizing adverse effects on batterysize and performance.

As a result of extensive and intensive research and various experiments,the present inventors have found that it is possible to efficientlyinhibit a temperature elevation during normal or abnormal operation ofthe battery and thereby to improve service life and safety thereof, whenmaterials undergoing phase change at a specific temperature and havinghigh latent heat of phase change are encapsulated into a capsule made ofan inert material and such a particulate capsule is incorporated intothe interior of a battery system by addition of the capsule to electrodeactive materials of the battery or application thereof to an inner orouter surface of the battery case. The present invention has beencompleted based on this finding.

Therefore, the battery system in accordance with the present inventionis configured such that the interior part of the battery structureincludes particles in which material having high latent heat of phasechange at a specific temperature is contained in a capsule made of aninert material.

The battery system in accordance with the present invention includesvarious kinds of primary and secondary batteries used in a wide varietyof electrical products including notebook computers, electric bicyclesand electric vehicles. In addition, such a battery system can be appliedto all kinds of batteries, regardless of shapes thereof includingcylinder-, square- and pouch-shapes.

As used herein, the term “specific temperature” refers to a temperaturethat may deteriorate performance and service life of the battery systemor threaten safety thereof in repeating state of specific numbers ortransient state. The specific temperature may be determined dependingupon the battery system. For example, the specific temperature ispreferably in the range of 0° C. to 120° C., more preferably 40° C. to120° C. and particularly preferably 50° C. to 100° C.

Materials having high latent heat of phase change at the specifictemperature (hereinafter, referred to as “phase change materials”) arethose materials that undergoes phase change, preferably from a solidphase to a liquid phase or vice versa, at the specific temperature, andhave latent heat greater than heat capacity/unit temperature of elementsconstituting the battery system. Single compounds, mixtures or complexesof various phase change materials may be employed. Phase change of suchmaterials includes the case in which phase change physically occurs atthe specific temperature, as well as the case in which a mixture of twoor more materials undergoes phase change via reversible chemicalreaction at the specific temperature.

Representative examples of phase change materials include, but are notlimited to, paraffin, polyethylene glycol, inorganic hydrates (forexample, Na₂HPO₄.12H₂O, Na₂SO₄.10H₂O and Zn(NO₃)₂.6H₂O). Among thesematerials, paraffin is particularly preferred as it has relatively highlatent heat, is inexpensive and the phase change temperature thereof iseasily modified by varying the average molecular weight thereof.

Where the phase change materials are directly added to structuralelements of the batteries, for example, where they are added toelectrode active materials, those materials undergo a phase change (forexample, a change from a solid phase to a liquid phase) at theabove-mentioned specific temperature and then escape into electrolytes,thus presenting problems associated with application thereof due toirreversible action mechanisms. In contrast, in accordance with thebattery system of the present invention, there is no occurrence of suchproblems since phase change materials are included in the interior ofthe battery under the state in which the phase change materials arecontained in a capsule made of an inert material.

The phase change material-containing capsule should be non-reactive withstructural elements of the battery, and should be made of materialscapable of maintaining phase change materials in a sealed state evenafter phase change of encapsulated phase change materials. Examples ofsuch inert materials include, but are not limited to, acrylic resins,melamine resins, urea resins and mixtures thereof. If necessary, thecapsule containing phase change materials may be made of materialsenabling decomposition or rupture of the capsule over a criticaltemperature. The critical temperature may be, for example a temperaturewhich may cause ignition or explosion of the battery. The thickness ofthe capsule containing phase change materials is not particularlylimited so long as it can exert effects in accordance with the presentinvention. The thickness of the capsule is preferably in the range of0.01 to 5 μm taking into consideration heat conductivity andmorphological stability of the capsule. Where the thickness of thecapsule is too thin, it is difficult to stably retain phase-changematerials. In contrast, where the thickness of the capsule isexcessively thick, it is undesirable in that heat conductivity islowered and the amount of phase change materials is relativelydecreased. In order to enhance heat conductivity of the capsule,materials exhibiting high heat conductivity may be further added toinert materials constituting the capsule, if desired.

As an example, particles including a capsule of inert material and phasechange materials contained in the capsule (hereinafter, referred to asphase-change particles), are prepared by further coating the outersurface of the phase-change particles (e.g., an outer surface of thecapsule) with conductive materials. The coated phase-change particlesare then included in the interior of the battery system. The conductivematerials may be various materials including metals, carbon black andconductive polymers. The outer surface of the phase-change particles maybe either entirely or partially coated. As examples of the conductivepolymers, polypyrrol, polyaniline, polyacetylene or derivatives thereofare preferably employed. As examples of such derivatives, mention may bemade of poly(3-butylthiophene-2,5-diyl),poly(3-hexylthiophene-2,5-diyl), poly(3-octylthiophene-2,5-diyl),poly(3-decylthiophene-2,5-diyl) and poly(3-dodecylthiophene-2,5-diyl).

Various techniques for coating certain particles with the conductivematerials are known in the art. In addition, preparation of phase-changeparticles coated with the conductive materials can also be carried outvia conventional methods well-known in the art.

Particularly, when added to electrode active materials, the conductivematerial-coated phase-change particles can also serve as a conductiveagent, and thereby provide effects that may replace or reduce therequired amount of conductive agents such as carbon black. Wherephase-change particles are coated with conductive materials having highheat conductivity such as metals, phase change materials can rapidlyrespond to changes in the external environment by enhancing heatconductivity of phase-change particles. In addition, when the conductivematerial-coated phase-change particles are employed for manganese-basedactive materials, it is possible to prevent high-temperature degradationand high-temperature volume expansion. Further, conductivematerial-coated phase-change particles are more desirable in batteriesin which high-rate characteristics are required.

Preferably, the phase-change particles have a particle diameter of about0.1 to 1000 μm. From the standpoint of exerting rapid reactivity withrespect to changes in temperature, small-diameter phase-change particleshaving a large surface area per unit weight are preferred. However, ifthe diameter of the corresponding phase-change particles is too small,problems associated with preparation of the phase-change particles anddifficulty associated with incorporation of the phase-change particlesinto the interior of the battery may be encountered. Therefore, thediameter of the phase-change particles can be suitably determined withinthe above-mentioned range.

There is no particular limit to positions that phase-change particlesare applied to the interior of the battery system. For example, thepositions to which the phase-change particles may be applied includecathode/anode active materials and/or current collectors, separators,electrolytes, inner surfaces and/or outer surfaces of battery cases,interior and/exterior parts of pouch battery cases and outer structuresof battery packs. If necessary, two or more structural elements of thebattery system or all elements thereof may be selected for applicationof phase-change particles.

For example, phase-change particles can be used in the form of particlesin electrode active materials or separators or electrolytes. Inaddition, phase-change particles are mixed with suitable solvents toprepare a slurry and then the resulting slurry can be applied in theform of a thin film to the inner surface and/or outer surface of batterycases or outer structures of battery packs, or may be formed into a filmwhich is then attached to the corresponding parts. In addition, uponmanufacturing pouch battery cases, phase-change particles can beincluded in the interior of the battery case by incorporatingphase-change particles into battery case materials. When they areincluded in electrolytes as additives, phase change particles may bepreferably prepared to have a small particle diameter to the extent thatparticles may pass through voids of the separator.

Phase-change particles in accordance with the present invention may becontained in or applied to the separators by which particles areincluded in the interior of the battery system.

As an example, it is possible to include phase-change particles in theinterior of the battery system without lowering battery capacity, byincorporating phase-change particles in the form of powder into amaterial of the separator at a manufacturing step of the separator of apolymer cell, or by incorporating phase-change particles into a gelpolymer coating solution, thereby being coated together with the gelpolymer, upon coating a gel polymer on the surface of the separator ofthe polymer cell. In this connection, Korean Patent Application No.2004-0038375, assigned to the present applicant, discloses a techniqueof coating the surface of the separator of the polymer cell with a gelpolymer, the disclosure of which is incorporated by reference herein inits entirety.

The content of phase-change particles may be determined depending uponvarious factors including kinds of phase change materials, a particlediameter of phase-change particles, kinds and shapes of batteries, andapplication sites of phase-change particles inside the battery system,and thus is not particularly limited. As an example, when phase-changeparticles are included in electrode active materials as additives, thecontent of phase-change particles is preferably in the range of 0.1 to10% by weight so as to exert desired effects of the present inventionwithout inhibiting functions of electrode active materials, based on thetotal weight of the active materials, but the present invention is notlimited thereto.

The processes for preparing phase-change particles are not particularlylimited so long as they are techniques for preparing particles having acore/cell structure as in the present invention. For example,phase-change particles can be prepared by dispersing phase changematerials in an aqueous phase via emulsification, followed bypolymerization on the surface of an oily phase in the resultingdispersion. Polymerization methods that can be used herein include, forexample, interfacial polymerization, in-situ polymerization, andcoacervation. Thus, encapsulation of phase change materials inaccordance with the present invention can be carried out using anymethod for general microcapsulation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1 and 2 schematically show a battery that can be used in thepresent invention;

FIG. 3 schematically shows a state in which phase-change particles areapplied to the surface of a battery case in accordance with oneembodiment of the present invention;

FIG. 4 shows a state in which an outer surface of a phase-changeparticle is coated with conductive material; and

FIG. 5 shows a state in which a conductive material is within aphase-change particle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 are schematic views of a lithium secondary battery, as abattery system to which phase-change particles in accordance with thepresent invention can be applied. Referring now to FIGS. 1 and 2, thelithium secondary battery is configured such that a thin film-likecathode and anode 10 and 30 are close to each other in betweenseparators 20 and 22, and are wound to form a stacked electrode assemblymounted inside the battery.

As the battery case, aluminum-laminated sheets and metal cans, which areemployed in conventional lithium polymer batteries, can be utilized.These battery case materials can be applied regardless of whether theinternal structure of the battery is a stacked or wound type.

Generally, a cathode 10 is prepared by applying a slurry containing acathode active material, a conductive agent and a binder to a currentcollector, followed by drying. An anode 30 is prepared by applying aslurry containing an anode active material, a conductive agent and abinder to a thin current collector (such as Cu foil or Ni foil),followed by drying. As the anode active material that can be utilized inthe present invention, mention may be made of crystalline carbon-basedmaterials such as natural and artificial graphite having a high degreeof graphitization, as well as amorphous carbon or carbon-based materialshaving a surface treated with amorphous carbon.

As electrolytes of lithium secondary batteries, electrolytes in whichlithium salts are dissolved in organic solvents can be used. As theorganic solvents, mixed solvents of ethylene carbonate (EC), propylenecarbonate (PC), gamma-butyrolactone (GBL), diethylcarbonate (DEC),dimethyl carbonate (DMC) and the like can be employed. As the lithiumsalts, it is preferred to use LiClO₄, LiAsF₆, LiPF₆, LiBF₄, CF₃SO₃Li andthe like.

FIG. 3 is a schematic view showing a state in which an application layer50 of phase-change particles is formed on an inner surface and/or outersurface of a battery case 40 in accordance with one embodiment of thepresent invention. The application layer 50 of phase-change particlesmay be formed partially or entirely on either or both of inner and outersurfaces of the battery case 40.

FIG. 4 shows a state in which an outer surface of a phase-changeparticle is coated with conductive material. As discussed above, aphase-change particle includes a capsule of inert material, and a phasechange material contained in the capsule. A conductive material may befurther on the outer surface of the phase-change particle (e.g., anouter surface of the capsule). Then, the coated phase-change particlemay be included in the interior of the battery system.

In contrast to FIG. 4, FIG. 5 shows a state in which a conductivematerial is within a phase-change particle. Specifically, the conductivematerial is between a capsule of inert material and a phase changematerial contained in the capsule, such that the phase-change materialis coated with the conductive material.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor the purpose of illustrating the present invention and should not beconstrued as limiting the scope and spirit of the present invention.

Example 1

1-1. Preparation of Cathode

93% by weight of LiCoO₂ as a cathode active material, 2% by weight ofphase-change particles, 2.5% by weight of Super-P (a conductive agent)and 2.5% by weight of PVDF (a binder) were added to NMP(N-methyl-2-pyrrolidone) as a solvent, thereby preparing a cathodemixture slurry, and the resulting slurry was coated on an aluminumcurrent collector to prepare a cathode. The phase-change particles(available from ENET Co., Ltd., Korea) contain encapsulated, saturatedparaffinic hydrocarbon having a melting point of 58° C., as a phasechange material. Latent heat of the phase-change particles was 145 J/g,based on the dry weight of the microcapsule.

1-2. Preparation of Anode

95.3% by weight of artificial graphite as an anode active material, 0.7%by weight of Super-P (a conductive agent) and 4% by weight of PVDF (abinder) were added to an NMP solvent, thereby preparing an anode mixtureslurry, and the resulting slurry was coated on a copper currentcollector to prepare an anode.

1-3. Preparation of Electrolyte

3% by weight of cyclohexylbenzene (CHB) was added to a solution of 1MLiPF₆ in EC/EMC, which was used as an electrolyte, to thereby prepare anelectrolyte for a lithium secondary battery.

1-4. Preparation of Battery

The cathode, porous separator and anode, as prepared hereinbefore, werewound into a roll form as shown in FIG. 1, which was then placed in asquare-shaped battery case to thereby prepare a battery as shown in FIG.2. That is, a porous separator 20 having a triple layer structure ofPP/PE/PP and a thickness of 20 μm (available from Celgard) is positionedbetween a cathode 10, prepared by coating a cathode active material onaluminum foil, and an anode 30, prepared by coating an anode activematerial on copper foil, which was then wound to a roll form andinserted into a cap, thereby preparing a square-shaped battery as shownin FIG. 2.

Example 2

A battery was prepared using the same procedure as in Example 1, exceptthat surfaces of phase-change particles were coated with carbon, and thecontent of carbon-coated phase-change particles used was 2.1% by weightand the content of Super-P (a conductive agent) was 2.4% by weight.Since the phase-change particles practically used were coated with 5% byweight of carbon, the content of phase-change particles alone was 2% byweight and the total content of conductive agent was 2.5% by weight whena slurry was prepared having the above-mentioned composition. Therefore,conditions for preparing the battery in this example are substantiallyidentical to those of Example 1.

Example 3

A battery was prepared using the same procedure as in Example 1, exceptthat phase-change particles were added to a separator instead of acathode active material and the content of the cathode active materialLiCoO₂ was 95% by weight.

Example 4

A pouch-type polymer battery was prepared using the same procedure as inExample 1, except that phase-change particles were incorporated into agel polymer solution prepared by dispersing 8% by weight of PVDF inacetone and the separator according to Example 1 was then subjected togel polymer coating using the resulting mixed solution, and the contentof the cathode active material LiCoO₂ was 95% by weight.

Example 5

A battery was prepared in the same manner as in Example 1, except thatphase-change particles were mixed with water instead of addition thereofto a cathode active material, and a water-soluble acrylic binder wasadded to the resulting mixture to thereby prepare a slurry which wasthen uniformly applied in the form of a thin film to an inner surface ofa square-shaped battery case, and the content of the cathode activematerial LiCoO₂ was 95% by weight.

Comparative Example 1

A battery was prepared using the same procedure as in Example 1, exceptthat phase-change particles were not added to a cathode active materialand the content of LiCoO₂ was 95% by weight.

Experimental Example 1

For batteries prepared in Example 1 and Comparative Example 1, eachbattery was charged to 4.2 V with 1 C at a temperature of 60° C. untilthe current at 4.2 V was 50 mA, and was discharged to 3 V at a rate of 1C. Such charge/discharge experiments were repeated 100 times and thenthe capacity rate of the battery relative to the initial capacity ratethereof was confirmed. As a result, the battery of Example 1 exhibited acapacity rate of 89.3% as compared to the initial capacity rate thereof,while the battery of Comparative Example 1 exhibited a capacity rate of64.5%. Therefore, it was confirmed that the battery system of thepresent invention can prolong the service life of the battery even whenoperated under high temperature conditions.

Experimental Example 2

For batteries prepared in Example 2 and Comparative Example 1, eachbattery was charged to 4.2 V with 1 C at a temperature of 25° C. untilthe current at 4.2 V was 50 mA, and was then discharged to 3 V at a rateof 0.5 C, 1 C and 2C, respectively. The capacity rate of the batteryrelative to 0.5 C discharge capacity was confirmed. As a result, thebattery of Comparative Example 1 exhibited the capacity rate of 100%,98.9% and 95.8%, while the battery of Example 2 exhibited the capacityrate of 100%, 99.0% and 96.2%. Therefore, when phase-change particleshaving surfaces coated with conductive materials were used, it wasconfirmed that the rate performance equal to or higher than use ofnon-coated phase-change particles was achieved even with reduced amountsof the conductive agent.

Experimental Example 3

Using respective batteries prepared in Examples 1 and 2 and ComparativeExample 1, safety tests (nail penetration tests after full charge of thebattery) were carried out. As a result, ignition and explosion were notobserved in batteries of Examples 1 and 2, while ignition and explosionoccurred in the battery of Comparative Example 1. Consequently, in thebattery system in accordance with the present invention, it wasconfirmed that rapid increase in battery temperature under abnormalconditions is inhibited by large latent heat produced upon phase-changeof phase-change materials contained in an inert capsule, thereby beingcapable of preventing ignition and explosion of the battery.

In addition, no ignition and explosion were observed in the safety testfor batteries of Examples 3 through 5.

INDUSTRIAL APPLICABILITY

As apparent from the foregoing, a battery system including phase-changeparticles in accordance with the present invention can prolong a servicelife of the battery by inhibiting temperature elevation inside thebattery under normal operating conditions without substantial effects onsize, shape and performance of the battery, and further, can inhibit therisk of explosion resulting from a sharp increase of temperature insidethe battery under abnormal operating conditions, thereby contributing tobattery safety.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A battery system in which an interior part of abattery structure comprises: phase-change particles including: a capsuleincluding an inert material; a phase-change material having a highlatent heat of phase change at a specific temperature, encapsulated inthe capsule; and a conductive material on an outer surface of thephase-change particles.
 2. The battery system according to claim 1,wherein the battery system is a lithium secondary battery.
 3. Thebattery system according to claim 1, wherein the phase-change materialis paraffin.
 4. The battery system according to claim 1, wherein thespecific temperature is in the range of 0° C. to 120° C.
 5. The batterysystem according to claim 4, wherein the specific temperature is in therange of 40° C. to 120° C.
 6. The battery system according to claim 1,wherein the conductive material is a metal, carbon black, or aconductive polymer selected from the group consisting of polypyrrol,polyaniline, polyacetylene and their derivatives.
 7. The battery systemaccording to claim 1, wherein the phase-change particles are included inone or more structural element selected from the group consisting of anelectrode active material, a current collector and a separator of abattery, an inner surface and/or outer surface of a battery case, aninterior and/exterior part of a pouch battery case and an outerstructure of a battery pack.
 8. The battery system according to claim 7,wherein the phase-change particles are incorporated into a material ofthe separator during manufacture of the separator.
 9. The battery systemaccording to claim 7, wherein, upon coating a gel polymer on the surfaceof the separator of a polymer cell, the phase-change particles areincorporated into a gel polymer coating solution, thereby being coatedtogether with the gel polymer.
 10. The battery system according to claim7, wherein the phase change particles are mixed with a solvent toprepare a slurry which is then applied or attached, in the form of afilm, to the inner surface and/or outer surface of the battery case,and/or the outer structure of the battery pack.
 11. The battery systemaccording to claim 1, wherein the phase-change particles have a particlediameter of 0.1 to 1000 μm.